1. Field of the Invention
The present invention relates to noise-suppressing turbofan nozzles and methods.
2. Prior Art
A principal component of the noise produced by a jet engine is jet noise, that is, noise from the turbulent mixing of a high velocity stream of gas flowing from a gas discharge nozzle of the engine into the ambient atmosphere.
One way to promote rapid mixing of a discharge flow with ambient air is to provide a multitube discharge nozzle, such as the nozzle shown in FIGS. 1 and 2 of U.S. Pat. No. 3,612,212. Alternatively, a multilobe nozzle, such as the nozzle shown in FIG. 3 of that patent, can be provided. An asymmetrical, multilobe mixing nozzle is shown in FIGS. 1, 2 and 3 of U.S. Pat. No. 3,174,282. A nozzle having a notch to promote mixing is shown in U.S. Pat. No. 3,968,853.
Another way to promote mixing is to provide a deflector for directing ambient air toward the discharge flow. Such a deflector can be in the form of an annular "aspirator ring", as shown in U.S. Pat. No. 3,174,282, or in the form of swingable "doors", as shown in U.S. Pat. No. 3,583,640.
In a conventional turbofan, jet noise can be reduced by mixing the primary exhaust flow, that is, the stream of air flowing through the turbofan compressor, combustor and turbine, with the flow of air bypassing these parts through the fan duct. According to the McGraw-Hill Encyclopedia of Science and Technology, combining the primary exhaust and fan flows results in a jet noise reduction of from 5 to 10 dB (see Vol. 1, page 207). Further noise reduction can be accomplished by promoting rapid and complete mixing of the combined flow with the ambient atmosphere.
One problem with suppressing jet noise by mixing a high temperature, high velocity flow with ambient air or a cooler flow of lesser velocity is that mixing changes the noise spectrum or "signature" by decreasing low frequency noise and increasing high frequency noise. Since high frequency noise can be more objectionable than low frequency noise, the overall noise level may be reduced without a corresponding reduction in the number of complaints to the noise produced by a jet engine.
U.S. Pat. No. 3,618,701 discloses another method of jet noise suppression which includes ejecting a "gaseous shield" below a jet engine discharge flow. However, this method requires complicated additional mechanism adding to the weight of an aircraft without appreciably increasing thrust.
Yet another method of suppressing jet noise is disclosed in U.S. Pat. No. 3,393,518 in which the high temperature primary exhaust flow of a turbofan
is discharged from the engine beneath at least the greater part of the stream of air which is passed through the fan, so that the noise in the stream of air tends to be refracted and/or reflected upwardly by the turbine exhaust gases.
See the abstract of U.S. Pat. No. 3,393,518. This patent is concerned with suppressing fan noise, whereas the present application is concerned with suppressing jet noise. As pointed out in U.S. Pat. No. 3,527,317, different methods must be used to suppress the different types of noise (see the paragraph beginning at column 1, line 47).